Actuator systems using arm assemblies enjoy extensive application in the field of data storage. Typical storage systems feature controlled elements, e.g., transducer heads, mounted on arm assemblies which are displaced by the actuator to enable the controlled element to access and/or record information in the storage medium. This type of storage system is particularly well-suited for optical and magnetic storage on disk-shaped or tape-type memory media.
Increasing densities of data stored in the media translate into higher track pitch which requires more precise actuator systems operating over a large bandwidth while maintaining stability and robustness. Present actuator systems experience several problems which limit their ability to satisfy these requirements.
The first set of problems is due to mechanical resonances of the arm structure. These vibrational modes include the natural modes of the actuator and those of any intervening mechanical components. With increasing mechanical complexity, the vibrational modes of any given actuator system become difficult to predict. The problem is further compounded as the operating frequency of the actuator system is increased. The vibrational modes limit the control loop gain and/or the bandwidth of the servo system. This causes the controlled element to experience excessive settling time after positioning, poor rejection of disturbances, poor tracking ability, or any combination of these.
The second set of problems is due to the high track pitch. Actuator systems capable of accessing closely spaced data tracks generally exhibit increased mechanical complexity. In particular, reduced tracking misregistration is often achieved with compound or dual-stage actuators. These types of actuators are commonly found in optical storage devices such as Compact Disks or DVD drives. Unfortunately, dual-stage actuators require a separate actuator for each stage. This translates into more electrical connections, greater operating voltages, a more complex control method and higher cost of fabrication and assembly. In addition, the mechanical complexity typically adds to the resonance related problems mentioned above.
To address the first set of problems, prior art systems have attempted to ensure stable operation of actuator systems by stabilizing the control loop. This has been done by inserting gain stabilizing filters such as electronic notch filters in the control loop path. These filters are placed generally in the downstream portion of the control loop to filter out the signal information within the band reject frequency range of the notch and thus help minimize excitation of these actuator vibrational modes.
Another technique for damping vibrational modes of a servo control system was presented by Masahito Kobayashi et al. in "MR-46 Carriage Acceleration Feedback Multi-Sensing Controller for Sector Servo Systems," at the International Conference on Micromechtronics for Information and Precision Equipment, Tokyo, Jul. 20-23, 1997. This proposed multi-sensing control system uses accelerometers to generate acceleration feedback. An acceleration feedback controller receives the feedback signals and compensates the servo to eliminate the mechanical resonance modes. Although Kobayashi's technique has been demonstrated, it can not be efficiently implemented without the use of notch filters. Furthermore, designing the feedback controller requires the designer to model the very complicated transfer function Hd(s) of the servo-controlled system.
The prior art also teaches gain stabilization through low-pass filtering in the control loop. In this approach the cutoff frequency of a low-pass filter that is inserted in the control loop is generally lower than the frequencies of any of the lightly damped resonances of the actuator structure. Thus, the components of the control signal at or near the resonance frequency are effectively prevented from exciting the vibrational modes of the actuator structure. Depending on the frequencies, this method helps ensure system stability. However, this approach also increases the phase shift at frequencies in the vicinity of the servo loop's unity gain crossing, thereby requiring the reduction of the bandwidth to maintain stability of the servo system. This drawback applies to gain stabilizing filters, including notch filters. The reduction in bandwidth and the reduction in the loop gain that generally accompanies it, in turn, reduces the ability of the servo system to correct errors and negatively affects tracking performance such as run out and other disturbances that are due to external excitation and non-linearities in positioning operations.
The problems associated with dual-stage actuator structure have not been sufficiently addressed by the prior art. Specifically, no presently known techniques allow the simplification of the dual-stage mechanical structure and the associated complexity of the control electronics.